Biocatalysts have gained increased importance in the direct recovery of primary and secondary metabolic substances.
Examples in actual practice which can be designated of technological interest for industry comprise:
1. Recovery of fructose from glucose through glucose isomerase.
2. Production of 6-APS from penicillin G through penicillin acylase.
3. Production of L-aspariginic acid from ammonium fumerate through E-coli.
4. Production of L-malic acid from fumerase with ammoniagene cells of brevi-bacterium.
In the technical microbiology, as well as in microbial engineering the term "biocatalyst" is understood to mean a biological system fixed through a macroscopic carrier, which is composed of enzymes or whole cellular microorganisms.
In recent times the use of fixed microorganisms as a biocatalyst has gained added importance and preference due to reduced production costs and improved process flexibility, especially with regard to multi-enzyme reactions.
The production of biocatalysts is essentially carried out by the physical encapsulation of the microorganisms in a polymer matrix.
Depending upon the production method and process design the following materials are used in the manufacture of such matrixes;
Polyacrylamide; Polymethacrylamide; Collagen; Cellulosetriacetate; Carboxymethylcellulose; Agar: Co-Poly-(maleic acid styrol); Carageenen. Biocatalysts produced according to the present state of technology are encumbered with difficulties in their manufacture and biocatalysts produced by such processes are burdened with deficient properties.
Particularly within the group: alginate, CMC, and Copolymer catalysts which are formed in a relatively simple manner by gel formation with multivalent cations, the lack of resistance to phosphate buffer solutions presents a considerable disadvantage wherein numerous reactions are being carried out in such a medium in actual practice.
In the use of natural electrolytes there can occur the danger of microbial attack, which can result in the destruction of the matrix material.
Furthermore, it will be hardly possible to employ these types of catalysts in solid bed reactors because of insufficient strength and tenacity.
The production of the cellulose triacetate catalyst through the intermediary of a wetspin process employs a fixation method which can only be used with few microorganisms because of the highly toxic nature of the utilized solvents, such as toluene or methylene chloride. Additionally, because of its fibrous form, the catalyst is restricted to use in solid bed reactors. The production of collagen catalysts is very complex. Because of the membrane form their use is restricted to spiral reactors. The toxic step due to hardening with glutardialdehyde can not be avoided. Polyacrylamide catalysts, on the other hand, which are the products of block polymerization, are described as sharp-edged, irregular granulates. Their use in stirring reactors evidences their high abrasion, which limits their capacity to be charged with microorganisms. This has been confirmed in the following German Laid-Open patent application:
______________________________________ DE-OS 2 252815: 4.8 g E. coli; = 120 ml catalyst 4%/vol. DE-OS 2 420102 17 g cells; = 170 ml catalyst 10%/vol. DE-OS 2 414128 12 g cells; = 120 ml catalyst 10%/vol. ______________________________________
Only a few processes have attained technical importance notwithstanding intense research. This is due to the following deficiencies: The mechanical strength and stability of many carriers is too low to allow the use thereof in large reactors; the fixation process is too complex which renders the use of such catalysts uneconomical; the attainable charging is insufficient, resulting in an unfavorable space/time yield.
One of the widest known processes employs physical encapsulation in a polymer matrix.
Examples are: polyacrylamide; polymethacrylamide; collagen, cellulose triacetate and ionotropic gels. These fixation processes are used because there occurs therein only relatively low deactivation of the enzymes, whole cells or cell fragments. It is known that the encapsulation in ionotropic gels represents a process in which, contrary to polyacrylamide or polymethacrylamide gels, practically non-toxic materials can be employed, as for instance, alginate, Ca.sup.2 . . . systems.
M. Kierstein and C. Bucke report in Biotechn. and Bioeng. 19 (1977) page 387, the encapsulation of enzymes, cell fragments and of whole cells in Ca-alginate gels.
These authors prepare the production of fibrous catalysts by injection of an Na-alginate solution into a Ca.sup.2+ precipitation bath. According to these authors the thus formed fibers exhibit only a marginal stability. A further disadvantage lies in the fact that the fibrous biocatalysts are restricted to their use in solid-bed reactors.
U. Hackel proposes in his dissertation (*) the formation of globular ionotropic gels through injecting the polyelectrolyte solution into a crosslinking bath. This author points out the utility of several other types of polyelectrolytes, including completely synthetic products such as styrolmaleic acid copolymer. FNT (*)Techn. Universitat Braunschweig 1976, Polymereinschluss von Mikroorganismenzu Aufbau und Reaktivitat von Biokatalysatoren.
However, these proposals have not lead to biocatalysts of sufficient strength and high content on biowetmass. The increase of strength by means of greater polymer content in the biocatalyst cannot be easily achieved because the highly viscous solution cannot be processed efficiently.